This invention relates to the magneto-optical measurement of currents in general and more particularly to an arrangement for the magneto-optical measurement of currents utilizing a rotation of the plane of polarization, dependent on the current to be measured, of a linearly polarized light beam, which is split into partial beams with orthogonal directions of polarization.
Measuring devices for currents in high voltage conductors and for large AC currents with a DC component wherein a light beam is fed via a polarizer and a magneto-optical measuring sensor, as well as through an analyzer, to a detector which is followed by an electronic circuit are known. The plane of polarization of the light beam is subjected to a rotation corresponding to the magnitude of the current in the measuring sensor, which is influenced by the magnetic field of the current to be measured. In an evaluator arranged at low-voltage potential, the magnitude of the rotation is converted, in an analyzer, into a corresponding intensity signal which can be picked up by a photo detector. The output signal of the detector is fed to the electronic circuit.
In one known arrangement, the evaluator contains an analyzer in which the light coming from the measuring sensor is split into two partial light beams, the planes of polarization of which are orthogonal and which change their intensity in opposite directions with the angle of rotation of the plane of polarization of the incident beam. The two partial light beams are fed to separate detectors which can preferably be semiconductor photo diodes and the outputs of which are fed to a differential amplifier. The difference voltage at the amplifier input is a measure for the Faraday rotation of the measurement signal. With this method, the noise components of the measurement signal, which are caused by intensity variations of the light beam, preferably a laser beam, can be eliminated. However, the photo diodes used have a small spread in their sensitivity. In addition, it is unavoidable, in a technical realization of the design, that the laser beams on the semiconductor photo cathode vary locally. These diodes have a production related location dependence of their photo sensitivity. Beam displacements of a few .mu.m can cause signal variations of up to several percent. The detectors associated with the two partial beams therefore furnish different signal variations which cannot be eliminated by the known differential measuring method (German Pat. No. 2,130,047).
Another known method (Rogers in "Optical Methods for Measurement of Voltage and Current at High Voltage", A.I.M., Leige, Traitment des donnes--1977, page 6, para. 3.2(c), Intensity Distribution Noise) therefore, works with a modulated light beam which is fed, via the measuring sensor and an analyzer, to a single photo diode, the output signal of which is processed in an electronic circuit.
The detector measures the intensity of the arriving measurement signal which also contains the superimposed modulation signal. Demodulation takes place in the following electronic circuitry. The intensity is influenced by the Faraday rotation in the measuring sensor as well as by the noise components, namely, the intensity fluctuations of the radiation source, as well as intensity variations which are caused by mechanical vibrations of the optical components in the ray path, and in addition, by the sensitivity of the photo diode itself. With this measuring method, the two intensity components can be separated from each other and the noise component on the measurement signal is suppressed. However, the linearity between the useful signal and the measurement signal is sufficient only at small angles of rotation up to about 2.degree.. For larger angles of rotation, with a correspondingly larger signal to noise ratio, a nonlinearity is obtained which depends on the signal amplitude.